1. Field of the Invention
The invention relates, generally, to the field of electrically controllable displays and, more particularly, to the field of electrochromic displays.
2. Description of the Prior Art
There are many uses for electrically controllable display devices. A number of such devices have been in commercial use for some time and include liquid crystal displays, light emitting diode displays, plasma displays and so on. Light emitting diode and plasma display panels are active, light emissive devices which require substantial power for their operation. In addition, it is difficult, if not impossible, to fabricate light emitting diode displays which are easily distinguishable under bright ambient illumination. Liquid crystal displays are operative only over a limited temperature range and have substantially no memory within the liquid crystal material. Further, the visibility of many liquid crystal displays decreases as the viewer moves a few degrees off axis.
Electrochromic displays have been developed which display information through a change in the color of portions in the display via electrochemical reaction of an active material to achieve a color change. In the familiar case of the tungsten oxide electrochromic, this color change is from white to blue. In the viologen electrochromics, the change is from white to violet. Because of their specific electrochemical mechanisms, such displays require substantial power and time to write or erase displayed information. The quantities of power required are undesirably large, especially for battery operation, and the time required to change displayed information makes such materials unacceptable for many display applications. None of these known displays provides more than a single color against a background.
Rare earth diphthalocyanines are known from prior publications to have electrochromic properties in which the color of the diphthalocyanine can change over a period of about eight seconds upon application of a potential difference across an electrochemical cell having a diphthalocyanine film on one of the electrodes. See for example P. N. Moskalev and I. S. Kirin, "Effect of the Electrode Potential on the Absorption Spectrum of a Rare-Earth Diphthalocyanine Layer", Opt. i Spektrosk, 29, 414 (1970), and P. N. Moskalev and I. S Kirin, "The Electrochromism of Lanthanide Diphthalocyanines" Russian J. Phys. Chem., 46, 1019 (1972). The diphthalocyanine does not require large amounts of power to change color, but the long period required for the color to change makes known diphthalocyanine performance characteristics unacceptable when measured against display requirements.
However, electrochromic display devices with more rapid switching characteristics are described in the U.S. Pat. No. 4,184,751 entitled "Phthalocyanine Electrochromic Display", by M. M. Nicholson. This display device includes an electrolyte of potassium chloride (KCl). This device has an operating temperature range of approximately 100.degree. to -11.degree. C. As a result many applications for using electrochromic displays are foreclosed because of the limited operational range.